/Engine/Plugins/Online/OnlineSubsystem/Source/Public/OnlineSessionSettings.h
Calculate the possible joinability state of this session check the values from left to right in order of precedence
true if the out params are valid, false otherwise
bool GetJoinability ( bool & bPublicJoinable, bool & bFriendJoinable, bool & bInviteOnly, bool & bAllowInvites ) const